Hook rulers have been around for ages. They're advantage is the ability to quickly and exactly reference the starting edge of a board. We've taken this idea and developed a simple hook that attaches to Woodpeckers Woodworking Rules. The Hook Stop is not compatible with older Woodworking Rules as well as any vintage 6". The way to know is to look at the top for two threaded holes located at 1/4-inch and 3/4-inch. If the threaded holes are there then the Hook Stop will attach. Rulers purchased after 12/1/2011 should have the holes in them.